"Robble robble" is the sound made by the Hamburglar, a supporting character in McDonald's McDonaldland. This is the land where Ronald McDonald, Mayor McCheese, and Grimace, among others, lived. The Hamburglar did not talk, bit would mumble and murmur. On a good day, these sounds could be interpreted as "robble robble".
The exact meaning of this utterance is highly context dependent.